Field of the Invention
The present invention relates to a focusing apparatus and method and in particular, but not exclusively, to a focusing apparatus and method for use in microscopic applications. The invention may also have various non-microscopic applications.
The term “microscopic” as used herein refers to optical applications having a resolution in the range of approximately 10−4 to 10−7 meter. Such applications are generally characterized by the use of high numerical aperture (NA) objective optics, typically with a NA in the range 0.5 to 0.95 in air, or 0.7 to 1.4 in immersion oil.
The invention may be used either in applications where light is collected from an object in order to form an image or detect certain properties of the object (for example wide field or scanning microscopy, fluorescence microscopy and so on), or in applications where light is focused onto an object in order to illuminate or affect the object in some way (for example micro-fabrication or laser surgery). It should be understood that any references herein to focusing and image formation apply equally to both types of application. The invention may also be used in applications where light is focused onto an object and an image of the object is then detected.
Description of the Related Art
In conventional microscopy, light from a specimen is collected by an objective lens and focused either by an ocular for viewing by eye or by an imaging lens onto a detector, for example a charge-coupled device (CCD). A typical arrangement is shown in FIG. 1. This includes an objective lens 2, an imaging lens 4 and a CCD detector 6. Light from a specimen 8 is collected by the objective lens 2 and focused by the imaging lens 4 onto the CCD detector 6. The image recorded by the CCD will represent a thin section 10 of the specimen. Light from all other parts of the specimen will be blurred out.
In a conventional arrangement, the image represents a two-dimensional plane (the X-Y plane) that is perpendicular to the optical axis 12 (the Z-axis) of the objective 2. Sometimes, however, it may be desirable to obtain a three-dimensional image or an image from a plane that is not perpendicular to the optical axis 12 of the objective 2. In either of these cases it is necessary to adjust the focal plane of the objective, so that it collects light from different regions of the specimen. A number of images obtained from different focal planes can then be combined to obtain either a 3D image or a 2D image in a non-perpendicular plane (for example the X-Z plane).